Metalloporphyrin complexes that phosphoresce at ambient temperatures have been used widely in photodynamic therapy (PDT), oxygen detection, and organic light emitting diodes (OLEDs). The most commonly used metalloporphyrins are d8 [primarily platinum(II), palladium(II), and gold(III)] and lanthanide complexes that emit at relatively long wavelengths (>600 nm) with lifetimes, in solution, in the 10-50 μs range under anaerobic conditions at room temperature. Although d6 metalloporphyrins are known to have desirable photophysical properties, they have found limited application. For example, ruthenium(II) porphyrins phosphoresce at room temperature at wavelengths longer than those of platinum(II) analogues, but have been less commonly used, due to oxidative instabilities. Porphyrin ligands are limited, however, in their emission intensity and their ability to stabilize metals in high oxidation states.
While metalloporphyrin complexes have been widely used, they typically stabilize metals in lower oxidation states than do corroles. Accordingly, first and second row transition metal corrole complexes have been investigated in an attempt to discover compounds capable of stabilizing metals in higher oxidation states. These corrole ligands are more electron-rich, and their strong sigma-donating nature enables them to stabilize high-valent metal centers that porphyrins cannot. Higher metal oxidation states strongly affect the redox properties of the chelated metal ion, and certain high-valent metal centers are desirable for catalysis. Additionally, corrole complexes produce more intense emission than metalloporphyrins.
These first and second row transition metal corrole complexes are, however, typically limited to fluorescence emission. In addition to their therapeutic uses, first row transition metal corroles function as good catalysts. For example, such corroles can be used in the activation of O2 by trivalent chromium, manganese, and iron; catalytic reduction of CO2 by iron(I) and cobalt(I); and iron(IV) aziridination of olefins. The redox processes of first and second row transition metal corrole complexes are, however, more often ligand-centered than metal-centered.